The use of rice really took off in the USA with the likes of the original Budweiser, which had the German immigrant brewers struggling with the local six row malts. Encyclopaedia Britannica 1911 edition has a bit to say on the American beers.
In America the common system of brewing is one of infusion mashing combined with bottom fermentation. The method of mashing, however, though on infusion lines, differs appreciably from the English process. A very low initial heat—about 100° F. —at which the mash remains for about an hour, is employed. After this the temperature is rapidly raised to 153-156° F. by running in the boiling "cooker mash," i.e. raw grain wort from the converter. After a period the temperature is gradually increased to about 165° F. The very low initial heat, and the employment of relatively large quantities of readily transformable malt adjuncts, enable the American brewer to make use of a class of malt which would be considered quite unfit for brewing in an English brewery. The system of fermentation is very similar to the continental "lager" system, and the beer obtained bears some resemblance to the German product. To the English palate it is somewhat flavourless, but it is always retailed in exceedingly brilliant condition and at a proper temperature. There can be little doubt that every nation evolves a type of beer most suited to its climate and the temperament of the people, and in this respect the modern American beer is no exception. In regard to plant and mechanical arrangements generally, the modern American breweries may serve as an object-lesson to the European brewer, although there are certainly a number of breweries in the United Kingdom which need not fear comparison with the best American plants.
I'm sure I read somewhere that they pushed beyond 50% with some brews.
Weyermann Pisner will convert up to 30% according to the Wey guy when I asked him at the lecture.
Edit: quite off topic, but the "somewhat flavourless" comment is interesting. The "party line" seems to be that American beers were full flavoured full bodied hoppy delights, that didn't survive Prohibition, and that when the breweries re-opened they produced the modern bland megaswill. The above article gives an interesting window into pre prohibition beers which suggests, to me anyway, that they weren't the malt and hop nirvana that many modern craft beer "campaigners" in the USA claim that they were.